1. Field of the Invention
The invention relates to a preloaded spring arrangement, in particular for spring loading office chair synchronizing mechanisms, and to a synchronizing mechanism that comprises such a spring arrangement.
2. Background Art
DE 199 22 446 A1 teaches a synchronizing mechanism for correlated seat/backrest motion of an office chair, in which a spring arrangement acts on the synchronizing mechanism in the direction of its non-tilted normal position. This spring arrangement comprises a pair of leg springs, which are located coaxially on a common axis, each having a supporting leg and a positioning leg. The positioning legs support themselves on an adjusting mechanism for modification of the preload of the leg springs. The adjusting mechanism is put into practice by a wedge-type sliding transmission which is adjustable crosswise of the longitudinal axis of the synchronizing mechanism and the driving wedge of which is adjustable by way of a spindle drive from the side of the synchronizing mechanism. The respectively associated positioning leg of the leg spring supports itself on this driving wedge.
Problems posed by the prior art spring load adjustment reside in the comparatively complicated construction of the adjusting mechanism on the one hand, which comprises a spindle drive and two wedges resting one upon the other for translation of the transverse displacement of the driving wedge occasioned by the spindle drive into a longitudinal displacement of the driven wedge. Since the supporting force of the positioning legs of the leg springs acts, via the driven wedge, directly on the contact surface between the two wedges, strong friction, in particular static friction, is found within the adjusting mechanism in particular in the case of high preloads. This may lead to the adjusting mechanism being comparatively hard to operate. Furthermore, upon preload regulation, both positioning legs are shifted simultaneously and by the same displacement so that sensitive regulation does not go without problems. Moreover, the restoring force of both leg springs must be countered, which implies an increase in energy requirements.
It is an object of the invention to embody a preloaded spring arrangement such that constructional simplicity is accompanied with the possibility of spring load regulation with great sensitivity and a decrease in energy requirements.
This object is attained by an adjusting unit of the type of an eccentric, which the two positioning legs are coupled with, and by which they are displaceable for regulation of the preload of the spring arrangement. For stepwise locked spring-load regulation, the eccentric adjusting unit comprises a pair of cams sitting axially side by side, the cam surfaces of which, related to the direction of rotation of the eccentric arrangement, successively exhibit plane locking sections, cam control sections and holding sections that are concentric of the axis of rotation of the eccentric arrangement. Finally, the respective locking, cam control and holding sections of the two eccentric cams are offset from one another in the direction of rotation of the eccentric arrangement in such a way that while one of the two positioning legs passes a cam control section that occasions displacement and thus load adjustment, the second positioning leg runs along the concentric holding section without experiencing any adjustment. Once the cam control section has been passed, the corresponding positioning leg applies on the locking section, which leads to a defined position of rotation of the adjusting unit. The other positioning leg has reached the beginning of the holding section so that, upon further rotation of the eccentric unit, it applies on the concentric holding section, occasioning no load counter to the adjusting rotation.
The construction according to the invention and in particular the eccentric adjusting unit, which comprises a pair of eccentric cams with functional sections that are displaced one relative to the other, help obtain actuation of the spring arrangement in fine steps of locking and with comparative ease during load adjustment. The construction according to the invention excels by extreme simplicity, there being only the need of joining a rotatable element to a turning handle for instance by way of a shaft.